1. Field of the Invention
The present invention relates to a golf ball and more particularly to an improvement in a spin performance of the golf ball.
2. Description of the Related Art
A golf ball hit with a golf club flies at an obliquely upward launch angle. The launch angle is caused by a loft angle of a head of the golf club. At the time of launch, the golf ball generates a so-called back spin. The back spin is caused by tangential force generated when the golf ball impacts the head having the loft angle. It has been reported that the amount of the back spin is almost proportional to an impulse of the tangential force generated during the impact (Dynamics and Design Conference ""98 in Hokkaido, Lecture Articles xe2x80x9cAnalysis of Spin Generating Mechanism in Impact of Golfxe2x80x9d).
After hitting, the golf ball flies in the air and falls after a while. A distance between the hitting and the falling is referred to as a carry. Usually, the golf ball rolls over the ground (a fairway, a green or the like) and stops the rolling. The distance between the falling and the stop is referred to as a run or a roll.
In the case of a tee shot, a great flight distance is desirable. Therefore, a golf ball providing a great carry and run is preferred. In the case of a shot aiming at a green (a shot made with an iron golf club in many cases), a golf ball having a small run is preferred. If the run is great, the golf ball falls from the green or the distance between a rest point and a cup is increased so that a subsequent putt is hard to perform. In other words, a golf ball to easily stop on the green is preferred for score-making.
The golf ball flies with a back spin. It tends to stop on the green more easily if a back spin rate is higher. The reason is that the back spin is a rotation in a reverse direction to a direction of a rotation of the rolling golf ball. From this viewpoint, a golf ball to have a higher back spin rate and to easily stop on the green has been developed in respect of a material and a structure.
For example, an attempt to increase a back spin rate has been made on a golf ball comprising a core and a cover by using a flexible material for the cover. Also in this method, however, a golf ball having a sufficient spin performance has not been obtained. If the cover is too flexible, there is a problem in that the cover is severely damaged by an impact on a club head at hitting or an impact on the ground at falling.
An attempt to easily apply a back spin by increasing a hardness of the core has also been made. Also in this method, however, a golf ball having a sufficient spin performance has not been obtained. If the hardness of the core is too high, there is a problem in that a hitting feeling is reduced.
In consideration of such circumstances, it is an object of the present invention to provide a golf ball having a higher back spin rate at hitting on the same conditions than that of a conventional golf ball.
In order to achieve the above-mentioned object, the present invention provides a golf ball in which a value of (T1/T2) is greater than 2.10 and is equal to or smaller than 2.50, wherein a direction of a counterclockwise rotation by 22 degrees with respect to a vertically upward direction is set to be a z direction, a direction of a counterclockwise rotation by 22 degrees with respect to a horizontally rightward direction is set to be an x direction, time series data on forces in the z and x directions which are applied to a load cell provided on a back face of a collision plate having a surface extended in the x direction when the golf ball impacts the collision plate in the vertically upward direction at a speed of 35 m/s are represented by Fn(t) and Ft (t) respectively, a time taken after a start of the impact before the Fn (t) is first changed from a positive number to zero is represented by T1, and a time taken after the start of the impact before the Ft(t) is first changed from a positive number to a negative number is represented by T2.
The golf ball has the value of (T1/T2) greater than 2.10 and equal to or smaller than 2.50 which is greater than that of a conventional golf ball. Therefore, an impulse of tangential force is increased during hitting as will be described below in detail. Therefore, the golf ball has a high back spin rate. In the case in which the golf ball falls into the green, a run is small. Also in the case in which the golf ball according to the present invention is hit with a middle iron or a long iron which generates a lower back spin rate than that of a short iron, the run can be controlled.